Power semiconductor devices are mainly used for power converters (DC-DC, AC-DC, DC-AC, and AC-AC), and high-frequency power amplifiers. Up until now, Si power semiconductor devices have been used widely. However, in recent years, it has been pointed out that the performance of Si power semiconductor devices can no longer be improved because of the material properties of Si.
Among properties required of power semiconductor devices, important are three properties, namely a high device withstand voltage, a low On resistance, and a low device capacitance. However, there is a trade-off relationship among these three properties, and when one is improved, the other two tend to deteriorate. This is the cause of the limitation in the improvement of the performance of power semiconductor devices using Si. In order to break through the limitation due to this trade-off, research and development is being promoted worldwide into power semiconductor devices using a wide band gap semiconductor.
In the present invention, a semiconductor satisfying (1) to (3) below is defined as a wide band gap semiconductor.
(1) A wide band gap semiconductor device is a semiconductor of which band gap energy is higher than that of Si (1.1 eV) and GaAs (1.4 eV). Specifically, it is a semiconductor of which band gap energy is 2 eV or higher.
(2) In terms of the composition of the elements forming the crystal, a wide band gap semiconductor is a semiconductor of which main components are boron (B), carbon (C), nitrogen (N), and oxygen (O), which are period 2 elements in the periodic table. Specifically, it is a semiconductor in which the ratio of the period 2 elements in all atoms constituting the crystal is ⅓ or higher.
(3) In terms of properties, a wide band gap semiconductor has a dielectric breakdown strength of 1 MV/cm or higher.
Specific examples of wide band gap semiconductors include silicon carbide, nitride semiconductors, oxide semiconductors, and diamond.
The chemical formula of silicon carbide (hereinafter referred to as SiC) is represented as SiC, and SiC has various polytypes. In particular, in the present specification, SiC means three kinds, namely 4H—SiC, 6H—SiC, and 3C—SiC.
Nitride semiconductors are compound semiconductors made of group III atoms (B, Al, In, and Ga), and nitrogen atoms (N). The total number of group III atoms is equal to the number of nitrogen atoms. The chemical formula of the nitride semiconductor is represented by the formula (1) below.BxAlyInzGa1-x-y-zN  (1)
In the formula, x, y, and z have values that satisfy 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≦1 In particular, GaN, InzGa1-zN (hereinafter, InGaN), AlyGa1-yN (hereinafter, AlGaN), and AlyInzGa1-y-zN (hereinafter, AlInGaN) are especially important as the materials of power semiconductor devices. AlN and BxAl1-xN (hereinafter, BAlN) have a band gap energy of 5 eV or higher, and can be used as a semiconductor and as an insulator at the same time.
Oxide semiconductors are semiconductors of which main component is oxygen atoms (O). Specific examples thereof include ZnO, Ga2O3, MgO, CdO, NiO, SnO2, Cu2O, CuAlO2, TiO2, VO2, In2O3, and SrTiO3. Two or more kinds of the oxide semiconductors may be combined to form a mixed crystal. A specific example thereof is ITO used as a transparent electroconductive film. Group II oxide semiconductors are especially effective as the materials of power semiconductor devices, and the chemical formula thereof is represented by the formula (2) below.ZnxMgyCd1-x-yO  (2)
In the formula, x and y have values that satisfy 0≦x≦1, 0≦y≦1, and x+y≦1.
Diamond is an insulator, and at the same time, behaves as a wide band gap semiconductor when a donor and an acceptor are added.
A particularly excellent physical property of the wide band gap semiconductors is a high dielectric breakdown strength. While the dielectric breakdown strength of Si is about 0.2 MV/cm, the dielectric breakdown strength of SiC (about 2 MV/cm), GaN (about 3 MV/cm), and diamond (from 5 to 10 MV/cm), which are wide band gap semiconductors, is about 10 times as high. Therefore, when wide band gap semiconductors are used as power semiconductor devices, the performance of the power semiconductor devices can be improved beyond the trade-off relationship among withstand voltage, On resistance, and device capacitance in the Si power semiconductor devices.
However, wide band gap semiconductor devices when used as power converters have a problem that the devices may be destroyed by a surge voltage. In an application as a power converter, it is when a wide band gap semiconductor device is turned off from an On state to an Off state that a surge voltage beyond the power supply voltage input to the power converter drops. A surge voltage may reach the device withstand voltage of the semiconductor device. In this case, an avalanche breakdown occurs in the semiconductor device, and the device is destroyed if the breakdown state continues.
Hence, wide band gap semiconductor devices need improvement in tolerance against breakdown. Here, tolerance against breakdown is defined as the maximum value of energy that a device can consume without being destroyed, when a voltage beyond the withstand voltage drops in an Off state and there flows a current in the device although it is in the Off state.
FIG. 1 shows a cross-sectional configuration diagram of a metal insulator semiconductor field effect transistor (hereinafter MISFET or insulated-gate field effect transistor) using SiC, as an example of a wide band gap semiconductor device according to a conventional technique. A metal oxide semiconductor field effect transistor (hereinafter, MOSFET) using SiO2 as a gate insulation film is one kind of MISFET.
Breakdown of a semiconductor device will be explained below, by taking the SiC-MISFET of FIG. 1 for example.
FIG. 2 shows a schematic diagram of a current-voltage characteristic of the SiC-MISFET shown in FIG. 1. As shown in FIG. 2, when a positive Vds is applied during an On state, there flows a drain current from a drain to a source. Here, Vds is an electric potential of a drain electrode with respect to an electric potential of a source electrode. On the other hand, when a Vds is applied during an Off state, there first flows a weak drain current that is attributed to a reverse leakage current in a body diode between a P-type region 222 and an N-type conductive region 203. After this, the Vds is raised, and when the Vds reaches a predetermined voltage Vava, an avalanche breakdown occurs, and the drain current increases sharply. As shown in FIG. 1, this avalanche breakdown current flows from the drain electrode 212 to the source electrode 210 along a breakdown current path 220.
Here, a device withstand voltage is a voltage at which a current that has started to flow during an Off state makes it no more possible for the Off state to be maintained. The withstand voltage of the SiC-MISFET of FIG. 1 is determined by the avalanche breakdown voltage Vava.
An avalanche breakdown is a phenomenon of a current flowing through a semiconductor device although the device is in an Off state, caused when en electric field strength in the semiconductor device reaches a value comparable to the dielectric breakdown strength of the semiconductor, and generation of electrons and holes becomes significant by avalanche multiplication. In the example of FIG. 1, generated holes migrate along the breakdown current path 220 and are eliminated from the source electrode 210, and electrons migrate along the breakdown current path 220 in a reverse direction and are eliminated from the drain electrode 212.
There are the following three factors due to which a wide band gap semiconductor device as a power converter is destroyed by an avalanche breakdown.
As a first factor, a surge voltage is more likely to occur in a wide band gap semiconductor device than in a Si power device. When a surge voltage above the device withstand voltage occurs, a breakdown state occurs. The magnitude of a surge voltage depends on a floating inductance (Ls) in the circuit and an amount of change of a drain current id per time (did/dt), and is proportional to Ls×did/dt. A wide band gap semiconductor device has a small device capacitance, and can be switched at a high speed. Therefore, it has a high did/dt value, and as a result, a surge voltage in the device is inherently high. Further, since the device capacitance is small, a surge voltage that occurs due to energy accumulated in the floating inductance is high, even if the accumulated energy is low. This is an unavoidable problem of a wide band gap semiconductor device that can be switched at a high speed.
As a second factor, a device is destroyed due to local concentration of an avalanche breakdown current in the device. In a wide band gap semiconductor device, an avalanche breakdown cannot occur uniformly throughout the device, and the breakdown current tends to concentrate. This problem is attributed to the fact that a P type and an N type of a wide band gap semiconductor cannot have low resistivities at the same time. Particularly, SiC, a nitride semiconductor, and an oxide semiconductor have a high resistivity when they are a P-type wide band gap semiconductor. Therefore, electrons and holes from inside the device, which are generated by avalanche multiplication, cannot be eliminated efficiently. As a result, a breakdown current concentrates at the location at which an avalanche breakdown started, and the device is destroyed at the location of the concentration.
As a third factor, a protective insulation film protecting the surface of a semiconductor device is destroyed. A dielectric breakdown strength of a wide band gap semiconductor is comparable to a dielectric breakdown strength of a protective insulation film such as SiO2 used commonly. Hence, when a strong electric field that would cause an avalanche breakdown is applied, a dielectric breakdown occurs in the protective insulation film not in the semiconductor.
A specific example of the destruction due to the second factor will be explained, by taking the SiC-MISFET of FIG. 1 for example. An electric field is applied to the body diode formed between the P-type region 222 and the N-type conductive region 203, and an avalanche breakdown occurs. Holes generated by the avalanche migrate along the breakdown current path 220, and are injected into a P-type contact region 206 and eliminated from the source electrode 210. At this moment, a diode between the P-type contact region 206 and an N-type contact region 205 is turned On by a voltage drop in the P-type region 222 and the P-type contact region 206 that have a high resistance. Due to this, electrons are injected from the source electrode 210 via the N-type contact region 205, to thereby further increase the breakdown current. As a result, the breakdown current concentrates at the predetermined location in the device, leading to a local destruction. That is, a MISFET, which is a unipolar device, incurs minority carrier injection and behaves as a bipolar device during a breakdown state. The current concentration in the device during a behavior as a bipolar device causes device destruction.
Due to the second factor, wide band gap semiconductor devices characterized in that the carriers carrying an On current during an On state are electrons have particularly outstanding avalanche breakdown destruction.
In FIG. 1, a reference sign 200 denotes a substrate, a reference sign 207 denotes an N-type contact region, a reference sign 211 denotes a gate electrode, and a reference sign 224 denotes a gate insulation film. In the present specification, the same reference numerals denote members having the same names.
As another specific example of a destruction due to the second factor, a heterojunction field effect transistor (hereinafter, HFET, or a heterojunction field effect transistor) using a nitride semiconductor will be explained. A cross-sectional configuration diagram of a nitride semiconductor HFET is shown in FIG. 3. As shown in this drawing, a nitride semiconductor HFET typically does not include a body diode formed of a PN junction. Hence, it does not include a path through which a breakdown current flows. In this case, it does not include a P-type region and an electrode for the P-type region through which holes generated by an avalanche are eliminated, which makes it harder for the holes to be eliminated. As a result, holes generated by avalanche multiplication are accumulated in the device. The hole accumulation induces electric field concentration, to thereby let an avalanche breakdown current flow locally concentratively in the device. Hence, the device is destroyed even by a weak avalanche current. In FIG. 3, a reference sign 103 denotes an N-type conductive region, a reference sign 110 denotes a source electrode, a reference sign 111 denotes a gate electrode, a reference sign 112 denotes a drain electrode, a reference sign 117 denotes a substrate electrode, a reference sign 124 denotes a gate insulation film, a reference sign 133 denotes a 2D electron gas, a reference sign 134 denotes a surface barrier layer, a reference sign 135 denotes a GaN foundation layer, a reference sign 136 denotes an initial growth layer, and a reference sign 137 denotes a heterogeneous substrate.
Notwithstanding the above said, there are also disclosed nitride semiconductor configurations including a body diode formed of a PN junction (NPL 1 and PTL 1). These devices are destroyed by concentration of an avalanche breakdown current due to a high resistivity in a P type, as is the SiC-MISFET of FIG. 1.
PTL 2 discloses a wide band gap semiconductor configuration having an improved tolerance against an avalanche breakdown. However, the fundamental problem of an avalanche breakdown in a wide band gap semiconductor described above is not resolved.
The problem of destruction of a wide band gap semiconductor device due to an avalanche breakdown in the device has been explained by taking the SiC-MISFET of FIG. 1 and the nitride semiconductor HFET of FIG. 3 for examples. However, destruction of a wide band gap semiconductor device due to an avalanche breakdown is a problem shared in common among various wide band gap semiconductor devices, which is attributed to a high surge voltage, a uniform avalanche breakdown in the whole device, and deterioration of an insulation film by a strong electric filed that would not cause an avalanche breakdown, as described above.
Specifically, unipolar and bipolar devices have the same problem. Here, a unipolar device is a semiconductor device that satisfies the following two conditions. As a first condition, it is a semiconductor device in which carriers to carry an On current to flow through main electrodes during an On state are either electrons or holes. As a second condition, at the moment, electrons or holes pass only an N-type semiconductor or a P-type semiconductor respectively in the semiconductor. The semiconductor devices of FIG. 1 and FIG. 3 are classified as unipolar devices in which carriers are electrons. Here, an N-type semiconductor and a P-type semiconductor include an inverted N-type layer and an inverted P-type layer at the interface between an insulation film and the semiconductor, respectively. Devices that do not satisfy the above two conditions are referred to as bipoar devices.
Here, main electrodes means source and drain electrodes of a field effect transistor, emitter and collector electrodes of a bipolar transistor, and cathode and anode electrodes of a diode.
More specifically, wide band gap devices described below have the same problem. A junction field effect transistor (hereinafter, JFET, or a junction-type field effect transistor), and a static induction transistor (hereinafter, SIT, or a static induction-type transistor), which are transistors classified as unipolar devices, have the same problem.
A bipolar transistor (hereinafter, BT, or a bipolar transistor), a heterojunction bipolar transistor (hereinafter, HBT, or a heterojunction-type bipolar transistor), and an insulated gate bipolar transistor (hereinafter, IGBT, or an insulated gate-type bipolar transistor), which are power transistors classified as bipolar devices, have the same problem.
Diodes also have the same problem; a Schottky barrier diode (hereinafter, SBD, or a Schottky barrier diode) and a junction barrier Schottky diode (hereinafter, JBSD, one variety of Schottky barrier diode), which are unipolar devices, and a P—N junction diode (hereinafter, PND) and a P-i-N junction diode (PiND), which are bipolar devices, have the same problem.
Unipolar devices have a switching speed higher than that of bipolar devices, and the problem of destruction due to an avalanche breakdown is more outstanding in the unipolar devices.
Further, the problem due to an avalanche breakdown is more outstanding in a lateral semiconductor device than in a vertical semiconductor device for the following two reasons. For a first reason, a lateral semiconductor device has a switching speed higher than that of a vertical semiconductor device, and incurs a high surge voltage as a result. For a second reason, high energy carriers generated by an avalanche breakdown flow through the surface of the semiconductor device and are injected into an insulation film protecting the surface of the semiconductor device, to thereby cause concentration of an electric field in the insulation film, leading to a destruction in the insulation film.
Here, a vertical device is a semiconductor device that has main electrodes on both sides of a semiconductor substrate, to thereby let an On current flow by penetrating through the semiconductor substrate. A lateral device is a semiconductor device that has main electrodes on either side of a semiconductor substrate. FIG. 1 and FIG. 3 show lateral semiconductor devices.